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Has BP Drilled Into an Asphalt Volcano?

As coastal residents along the Gulf are on high alert after the BP fiasco, emerging evidence suggests that the drill site is an asphalt volcano, a rare type of submarine volcano (seamount) first discovered in 2003.

While volcanism is common under the sea, nobody dreamed we would eventually drill into one. In 2003, a research expedition in the Gulf of Mexico found something on a seafloor hill the scientists named Chapopote, the Mexican Spanish name for tar. It was the world's first known asphalt volcano. There are more being found all the time.

The research team, led by Ian MacDonald of Texas's A&M University, documented tar flows as wide as 20 m (66 ft) across. Also discovered alongside the asphalt were areas soaked with petroleum and with methane hydrate, also spewed from the volcano. This kind of an environment proves attractive to chemical-loving bacteria and tubeworms, although the exact biogeochemical relationship is not yet known.

The tar is relatively hot when it comes out of the seafloor, but just like undersea lava flows, it is quickly cooled by the much colder seawater around it. This produces forms similar to the distinctive A'a and pahoehoe types of basalt lava flow seen in places like Hawaii. Another similarity is that the tar heats methane hydrate and causes it to explode into a free gas, similar to the action hot lava has on groundwater in phreatomagmatic eruptions.

The team proposed an asphalt volcano formation theory in a paper published in Eos.The article suggested that water heated past the critical point underneath the seafloor found a passageway to the surface, most likely a salt dome, and carried with it a heavy load of hydrocarbons and dissolved minerals. A special property of such critically heated water is that it can mix with oils, whereas normal water cannot. The same process is attributed to the formation of black smokers. Once the water reaches the surface, it cools, and its carrying capacity drops. The lighter compounds in the mixture escape to the surface, while the tar and other heavier materials remain on the seafloor, eventually building up the asphalt volcano's structure.

Two dives with in 2009 and a detailed photographic survey of the area by the autonomous underwater vehicle Sentry showed many similarities to volcanic flows, including flow texture and cracking of the asphalt layers. Carbon dating puts the structures at between 30 and 40 thousand years old. They had at one time been a prolific source of methane, a greenhouse gas. The two largest structures, less then 1 km (1 mi) apart, are pocketed by pits and depressions, a sign of methane gas bubbling up long ago. Researchers wonder if the gas contributed in the past to the most recent ice age. Although the structures are still emitting residual gas, at present the amounts are too small to have any effect. The amount of crude oil in the largest of the structures alone is "enough to fuel my Honda Civic for about half a billion miles. [However] the quality of the material is very poor...It's not worth something like light sweet crude," said Valentine. The petroleum in the structure is more viscous than that which is usually found in underground wells. This is because it has had less time to "bake" under the Earth's heat before being released. In addition, as much as 20% of its mass is made of "junk"—microscopic organisms, sand, and miscellaneous materials that gradually accumulated in the oil.

Analysis of the samples collected from the mounds suggest that they required several decades, even centuries, to build up their current bulk, and that the volcanoes last erupted around 35,000 years ago. In addition they may account for a mysterious spike in oceanic methane concentrations around 35,000 years ago. Methane forms naturally alongside the petroleum underneath the structure, and while petroleum flows have long abated, some residual methane continues to bubble up. This burst of methane would have caused a rapid increase in the population of methane-eating bacteria, which in turn caused a decrease in oxygen in the water, possibly causing a dead zone, in addition to the large amounts of crude oil released into the environment.

Ecologically speaking, the presence of these structures provides a hard surface on which life can grow, as the surrounding ocean floor is generally muddy. This is similar to what happens on seamounts, resulting in their place as an ecological "hub."



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